1. Field of the Invention
The present invention relates to an integrated circuit device for a power supply, connected to an AC power supply via an input circuit having a capacitor, and relates in particular to an integrated circuit device for a power supply, capable of reliably discharging a charge that has accumulated in the capacitor during interruption of the AC power supply.
2. Description of the Related Art
Integrated circuit devices for power supplies, which are connected to an AC power supply via an input circuit having a capacitor, are often used as control mechanisms for switching power devices that generate a specific DC voltage from the AC power supply. FIG. 4 is a schematic configuration diagram of a common, conventional switching power device, in which 1 is a switching power supply body formed primarily of an IGBT, MOS-FET or other switching element (not shown), and 2 is an integrated circuit device for a power supply (power IC), which performs PWM control of the switching power supply body 1.
In the figure, 3 is an input circuit constituting the input filter for the AC power supply ACin, and 4 is a rectifying circuit that performs full-wave rectification of the output from the input circuit 3 and supplies it to the switching power supply body 1. The power IC2 operates by rectifying the output (AC power supply ACin) of the input circuit 3 via rectifying diodes D1 and D2 during initial power activation, and then inputting it into a terminal VH to activate a start-up circuit 5. After the switching power supply body 1 has been activated, the power IC2 operates by inputting the output obtained from the switching power supply body 1 into a terminal VCC.
The power IC2 is itself provided with a J-FET (constant current diode) 6, which converts the power supplied to the terminal VH into constant current to activate the start-up circuit 5, and with a zener diode (constant voltage diode) 7, which converts the voltage applied to the terminal VCC into constant voltage and inputs it. The power IC2 operates by producing a specific internal supply voltage VDD from either the output of the start-up circuit 5 or the voltage applied to the terminal VCC, and supplying this internal supply voltage VDD to a PWM control circuit or the like (not shown).
The PWM control circuit performs on/off control of the switching power supply body 1, specifically the switching element, in accordance with information input to the respective terminals FB, CS and LAT. Various systems have previously been proposed for the PWM control circuits in the switching power supply body 1 and power IC2, but they do not directly relate to the purport of the invention.
This kind of switching power device is also provided with a discharge circuit for discharging the charge that accumulates in the capacitor Cx of the input circuit 3 when the AC power supply ACin is interrupted (when the power is off). Specifically, as shown in FIG. 4, a discharge resistor Rx is connected parallel to the capacitor Cx. However, when the input circuit 3 is provided with a discharge resistor Rx as a discharge circuit, there is a risk of power loss from the discharge resistor Rx.
Therefore, it has been proposed that an electronic discharge circuit 8 be incorporated into the power IC2 in place of the discharge resistor Rx as shown in FIG. 5 for example, in order to prevent power loss from the discharge resistor Rx (see, for example, US Patent No. 2010/0309694 (Specification) and WO 2012/033120). With so-called IC discharge such as this electronic discharge circuit 8, it is possible to keep the loss to roughly zero (0) and increase the efficiency of the switching power device regardless of the capacity of the capacitor Cx as shown in FIG. 6.
However, because the electronic discharge circuit 8 shown in FIG. 5 operates in response to the internal supply voltage VDD of the power IC2, it ceases to function if the internal supply voltage VDD is lost. This means that it may not be possible to discharge the charge that has accumulated in the capacitor Cx if the AC power supply ACin is interrupted. The most common reason for loss of the internal supply voltage VDD is failure of the terminal VCC as shown in FIG. 7.